diff --git a/lib/rack/test/utils.rb b/lib/rack/test/utils.rb
index d1f74568..09ff2e25 100644
--- a/lib/rack/test/utils.rb
+++ b/lib/rack/test/utils.rb
@@ -74,16 +74,10 @@ def normalize_multipart_params(params, first=false)
 
           case value
           when Array
-            value.map do |v|
-              if v.is_a?(Hash)
-                nested_params = {}
-                normalize_multipart_params(v).each do |subkey, subvalue|
-                  nested_params[subkey] = subvalue
-                end
-                (flattened_params["#{k}[]"] ||= []) << nested_params
-              else
-                flattened_params["#{k}[]"] = value
-              end
+            next if value.empty?
+
+            flattened_params["#{k}[]"] = value.map do |v|
+              v.is_a?(Hash) ? normalize_multipart_params(v) : v
             end
           when Hash
             normalize_multipart_params(value).each do |subkey, subvalue|
@@ -106,10 +100,14 @@ def build_parts(buffer, parameters)
       # Append each multipart parameter value to the buffer.
       def _build_parts(buffer, parameters)
         parameters.map do |name, value|
-          if name =~ /\[\]\Z/ && value.is_a?(Array) && value.all? { |v| v.is_a?(Hash) }
-            value.each do |hash|
+          if name =~ /\[\]\Z/ && value.is_a?(Array) && value.any? { |v| v.is_a?(Hash) }
+            value.each do |item|
               new_value = {}
-              hash.each { |k, v| new_value[name + k] = v }
+              if item.is_a?(Hash)
+                item.each { |k, v| new_value[name + k] = v }
+              else
+                new_value[name] = item
+              end
               _build_parts(buffer, new_value)
             end
           else
diff --git a/spec/rack/test/utils_spec.rb b/spec/rack/test/utils_spec.rb
index 090276d3..c11c52dc 100644
--- a/spec/rack/test/utils_spec.rb
+++ b/spec/rack/test/utils_spec.rb
@@ -148,6 +148,21 @@ def respond_to?(m, *a)
     params['files'][1][:tempfile].read.must_equal "baz\n"
   end
 
+  it 'builds mixed hash and primitive arrays without mutating them' do
+    values = [{'id' => '1'}, 'two', {'id' => '3'}].freeze
+    data = Rack::Test::Utils.build_multipart('items' => values)
+
+    options = {
+      'CONTENT_TYPE' => "multipart/form-data; boundary=#{Rack::Test::MULTIPART_BOUNDARY}",
+      'CONTENT_LENGTH' => data.length.to_s,
+      :input => StringIO.new(data)
+    }
+    params = Rack::Multipart.parse_multipart(Rack::MockRequest.env_for('/', options))
+
+    params['items'].must_equal [{'id' => '1'}, 'two', {'id' => '3'}]
+    values.must_equal [{'id' => '1'}, 'two', {'id' => '3'}]
+  end
+
   it 'builds multipart bodies from mixed array of a file and a primitive' do
     files = [Rack::Test::UploadedFile.new(multipart_file('foo.txt')), 'baz']
     data = Rack::Test::Utils.build_multipart('files' => files)